Duplex automatic document feeder and duplex document scanning method using the same

ABSTRACT

A duplex automatic document feeder includes first, second, third, and fourth paths, an ejecting path, a scanning module, first and second reversing roller units, a feeding tray, and an ejecting tray. A document is moved from the feeding tray to the scanning module for scanning of a first side surface thereof. Subsequently, the document is moved along a second-side scanning path defined by the first reversing roller unit, the third path, a portion of the first path, the fourth path, and the first reversing roller unit to allow for scanning of a second side surface thereof. After the first and second side surfaces are scanned, the document is moved along a document-inverting path defined by the second reversing roller unit, a portion of the fourth path, the ejecting path, a portion of the second path, the first reversing roller unit, and the ejecting tray.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Continuation-in-Part (CIP) of U.S. patent application Ser. No.11/699,374, filed on Jan. 30, 2007. This application also claimspriority of Taiwanese Application No. 096129815, filed on Aug. 13, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a duplex automatic document feeder and aduplex document scanning method using the same, and more particularly toa duplex automatic document feeder applicable to a copy machine, and aduplex document scanning method using the same.

2. Description of the Related Art

Referring to FIG. 1, a compact auto-document feeder 1 disclosed in U.S.Pat. No. 5,784,680 includes a document setting tray 11, a scanningmodule 12, an ejecting tray 13, and a switching gate 14. A firsttransfer path 101 extends from the document setting tray 11 to thescanning module 12. A second transfer path 102 extends from the scanningmodule 12 to the ejecting tray 13. A third transfer path 103 extendsfrom a rear end of the second transfer path 102 to the first transferpath 101. The switching gate 14 is disposed among the second, third, andfourth transfer paths 102, 103, 104. Operations of the feeder 1performed on one document will be described hereinafter.

When the feeder 1 is operated in a simplex document-feeding mode, thedocument is fed from the document-setting tray 11, and is moved throughthe first transfer path 101 and the scanning module 12 for scanning ofthe document. Subsequently, the document is moved onto the ejecting tray13 through the second transfer path 102 with the switching gate 14 beingdisposed in a first position shown by the solid lines in FIG. 1.

When the feeder 1 is operated in a duplex document-feeding mode, thedocument is moved from the first transfer path 101 onto the secondtransfer path 102 for scanning of a first side surface thereof. At thesame time, the switching gate 14 is pivoted to a second position shownby the phantom lines in FIG. 1. Hence, the document is moved from thesecond transfer path 102 onto the fourth transfer path 104 andsimultaneously interacts with a roller unit 16. Thereafter, therotational directions of rollers of the roller unit 16 are changed so asto return the document to the first transfer path 101 via the thirdtransfer path 103. Thus, the document is moved from the first transferpath 101 onto the second transfer path 102 once again so as to allow forscanning of a second side surface thereof. After completion of thescanning of the first and second side surfaces of the document, in orderto enable ejection of the document with the first side surface of thedocument facing downwardly, the document must be further moved through adocument-inverting path defined by the third transfer path 103, adownstream portion of the first transfer path 101, and the secondtransfer path 102 so as to drop from the downstream end of the secondtransfer path 102 onto the ejecting tray 13.

The feeder 1 has the following disadvantages:

-   1. The document-inverting path is long, thereby reducing the    operating efficiency of the feeder 1.-   2. Referring to FIG. 2, before a trailing end 151 of the document is    moved from the roller unit 16, a leading end 152 of the document may    be moved onto the roller unit 16, thereby resulting in occurrence of    a paper jam.

To solve the paper jam problem, in US Patent Application Publication No.2002/0054382 and U.S. Pat. No. 6,307,614, the roller unit 16 is replacedwith two roller units each defining a nip.

Referring to FIG. 3, a duplex automatic document feeder 2 disclosed inUS Patent Application Publication No. 2002/0054382 includes first,second, third, and fourth document-feeding paths 201, 202, 203, 204, ascanning module 22, a middle roller 23, an upper roller 24, and a lowerroller 25. Relative positions among the first, second, and thirddocument-feeding paths 201, 202, 203, as well as the scanning module 22are similar to those of U.S. Pat. No. 5,784,680. The fourthdocument-feeding path 204 is U-shaped, and has upper and lowerhorizontal sections 205, 206, and a curved section 207. The middleroller 23 is disposed between the upper and lower horizontal sections205, 206. The upper and lower rollers 24, 25 are in frictional contactwith the middle roller 23, and are respectively adjacent to the upperand lower horizontal sections 205, 206 of the fourth document-feedingpath 204. After completion of a second scanning operation, the documentis moved from the second document-feeding path 202 onto the lowerhorizontal section 206 of the fourth document-feeding path 204. When thedocument is moved into a space between the middle roller 23 and thelower roller 25, the rotational directions of the middle roller 23 andthe lower roller 25 are changed so as to allow the document to be movedonto the curved section 207 and the upper horizontal section 205 of thefourth document-feeding path 204, thereby passing through a spacebetween the middle roller 23 and the upper roller 24. Finally, thedocument is moved from the middle roller 23 and the upper roller 24 ontoa discharging tray (not shown).

The lower roller 25 is movable relative to the middle roller 23 betweenupper and lower positions shown respectively by the solid and phantomlines in FIG. 3 through operation of an elevating mechanism (not shown).When the lower roller 25 is moved to the lower position, two documentscan be moved in two opposite directions through the space between themiddle roller 23 and the lower roller 25, thereby preventing occurrenceof a paper jam.

However, the structure of the feeder 2 is complex due to the presence ofthe elevating mechanism. This increases the manufacturing costs of thefeeder 2.

SUMMARY OF THE INVENTION

The object of this invention is to provide a duplex automatic documentfeeder and a duplex document scanning method using the same that canovercome the abovementioned disadvantages associated with the prior art.

According to an aspect of this invention, there is provided a duplexautomatic document feeder adapted for use in an image-formation device.The image-formation device has a scanning module for scanning adocument. The duplex automatic document feeder comprises:

a feeding tray adapted for feeding the document;

a first path adapted to extend from the feeding tray to the scanningmodule so as to invert the document when the document is moved along thefirst path;

a feeding/separating roller adapted for moving the document from thefeeding tray onto the first path;

an ejecting tray;

a first reversing roller unit disposed in proximity to the ejecting trayand adapted for allowing the document to move in a direction onto theejecting tray and in an opposite direction to reverse the document;

a second path adapted to extend from the scanning module to the firstreversing roller unit and adapted for guiding movement of the documentjust after a first scanning operation is performed on the document;

a third path in spatial communication with the first and second pathsand adapted for guiding movement of the reversed document from the firstreversing roller unit onto an intermediate portion of the first path;

a second reversing roller unit disposed in proximity to the firstreversing roller unit and adapted for reversing the document;

a fourth path in spatial communication with the second path and thesecond reversing roller unit and adapted for guiding movement of thedocument just after a second scanning operation is performed on thedocument;

an ejecting path adapted for guiding movement of the document from thesecond reversing roller unit onto an intermediate portion of the secondpath; and

a first guide unit adapted to be controlled to guide movement of thedocument along a selected one of the second and fourth paths.

According to another aspect of this invention, a duplex documentscanning method comprises the steps of:

-   (a) moving a document along a first path to pass through a scanning    module so as to perform a scanning operation on a first side surface    of the document;-   (b) moving the document along a second path into a first reversing    roller unit;-   (c) driving the first reversing roller unit to reverse and move the    document from the first reversing roller unit onto the first path    via a third path to thereby pass again through the scanning module    so as to perform a scanning operation on a second side surface of    the document opposite to the first side surface;-   (d) moving the document along a fourth path into a second reversing    roller unit; and-   (e) driving the second reversing roller unit to reverse and move the    document from the second reversing roller unit into the first    reversing roller unit via a portion of the fourth path, an ejecting    path, and a portion of the second path onto an ejecting tray.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of this invention will becomeapparent in the following detailed description of the preferredembodiments of this invention, with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic side view of a compact auto-document feederdisclosed in U.S. Pat. No. 5,784,680;

FIG. 2 is a schematic side view of the compact auto-document feeder ofFIG. 1, illustrating how a paper jam may occur between second and thirdtransfer paths;

FIG. 3 is a schematic side view of a duplex automatic document feederdisclosed in US Patent Application Publication No. 2002/0054382;

FIG. 4 a is a fragmentary schematic side view of the first preferredembodiment of a duplex automatic document feeder according to thisinvention in a state of use with a copy machine;

FIG. 4 b is a schematic side view of the first preferred embodiment,illustrating a first-side scanning path;

FIG. 4 c is a schematic side view of the first preferred embodiment,illustrating a second-side scanning path;

FIG. 4 d is a schematic side view of the first preferred embodiment,illustrating a document-inverting path;

FIG. 4 e is a schematic side view illustrating a modification to anejecting path of the first preferred embodiment;

FIG. 5 a is a schematic side view of the second preferred embodiment ofa duplex automatic document feeder, illustrating a first-side scanningpath;

FIG. 5 b is a schematic side view of the second preferred embodiment,illustrating a second-side scanning path;

FIG. 5 c is a schematic side view of the second preferred embodiment,illustrating a document-inverting path;

FIG. 5 d is a schematic side view illustrating a modification to firstand second reversing roller units of the second preferred embodiment;

FIG. 5 e is a fragmentary schematic side view of the second preferredembodiment, illustrating a junction between second and third paths;

FIG. 6 a is a schematic side view of the third preferred embodiment of aduplex automatic document feeder, illustrating a first-side scanningpath;

FIG. 6 b is a schematic side view of the third preferred embodiment,illustrating a second-side scanning path;

FIG. 6 c is a schematic side view of the third preferred embodiment,illustrating a document-inverting path;

FIG. 6 d is a schematic view of a first guide unit of the thirdpreferred embodiment; and

FIG. 6 e is a schematic side view illustrating a modification to firstand second reversing roller units of the third preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before the present invention is described in greater detail inconnection with the preferred embodiments, it should be noted thatsimilar elements and structures are designated by like referencenumerals throughout the entire disclosure.

The duplex automatic document feeder of this invention is applicable toa copy machine, a fax machine, a scanner, or other image formingdevices. In each of the following embodiments, this invention isdescribed in an exemplary application to a copy machine.

Referring to FIGS. 4 a and 4 b, the copy machine 6 includes a top cover61 and a scanning module 32. The first preferred embodiment of a duplexautomatic document feeder according to this invention includes a housingunit defined by the top cover 61; a feeding tray 311 disposed in thehousing unit; an ejecting tray 30; a pair of first and second reversingroller units 331, 332 disposed in proximity to the ejecting tray 30; apath assembly in spatial communication with the feeding tray 311, theejecting tray 30, and the scanning module 32 and including a first path301, a second path 302, a third path 303, a fourth path 304, and anejecting path 305; a guide assembly including first, second, third, andfourth guide units 34, 45, 46, 47; a control module (not shown); and asensor assembly including first, second, and third sensors 41, 42, 43.The scanning module 32 is disposed under the feeding tray 311. The firstpath 301 extends from the feeding tray 311 to the scanning module 32.Two transfer roller units 51, 52 are disposed along the first path 301and in proximity to the scanning module 32 and the feeding tray 311,respectively. The first sensor 41 is disposed along the first path 301between the transfer roller unit 51 and the scanning module 32. A stackof documents (not shown) to be scanned can be placed on the feeding tray311 such that the uppermost document (referred to as “the document”hereinafter) is in frictional contact with the feeding/separating roller312. The feeding/separating roller 312 is driven by a motor (not shown)to move the document onto the first path 301. Through operation of thetransfer roller units 51, 52, the document is moved from the first path301 onto the second path 302 via the scanning module 32. When thedocument is moved along the first path 301, it is inverted. When thedocument is passed through the scanning module 32, it is scanned.

The second path 302 has a front end 3021 in spatial communication with adownstream or lower end of the first path 301, and a rear end 3022 inspatial communication with the first reversing roller unit 331. Thefirst reversing roller unit 331 is disposed in proximity to and betweenthe ejecting tray 30 and the rear end 3022 of the second path 302. Thefirst reversing roller unit 331 includes a middle roller 333 and anupper roller 334 flanking the rear end 3022 of the second path 302. Theupper roller 334 is disposed above and in frictional contact with themiddle roller 333 to define a nip therebetween. The middle roller 333can be driven by a motor (not shown) of the control module to rotateclockwise to thereby move the document onto the ejecting tray 30, orcounterclockwise to reverse the document. The third sensor 43 isdisposed in proximity to the rear end 3022 of the second path 302 andthe first reversing roller unit 331 so as to output a signal foractivation of the first reversing roller unit 331 to rotate just beforethe document is moved into the first reversing roller unit 331.

The third path 303 is disposed between the feeding tray 311 and thesecond path 302, and has a front end in spatial communication with anintermediate portion of the first path 301, and a rear end in spatialcommunication with a portion of the second path 302 adjacent to the rearend 3022. As such, the first path 301 has an upper path portion 301′disposed above the front end of the third path 303, and a lower pathportion 301″ disposed below the front end of the third path 303.

The second guide unit 45 is disposed at a junction between the secondand third paths 302, 303, and is configured as an elongated Mylar platehaving a fixed end and a movable end that is disposed at a closeposition shown in FIG. 4 c when no external force is applied thereto,and that is pushable to an open position shown in FIG. 4 b. The secondpath 302 is divided by the second guide unit 45 into a front pathsection 302′ distal from the first reversing roller unit 331, and a rearpath section 302″ proximate to the first reversing roller unit 331, asshown in FIG. 4 c. As such, the second guide unit 45 can preventmovement of the document from the rear path section 302″ to the frontpath section 302′ along the second path 302 while allowing for movementof the document from the front path section 302′ to the rear pathsection 302″ along the second path 302. The third and fourth guide units46, 47 are similar to the second guide unit 45 in construction.

The fourth path 304 has a front end in spatial communication with aportion of the second path 302 adjacent to the front end 3021 of thesecond path 302, and a rear end in spatial communication with the secondreversing roller unit 332. The second reversing roller unit 332 includesthe middle roller 333 and a lower roller 335 flanking the rear end ofthe fourth path 304. The lower roller 335 is disposed under and infrictional contact with the middle roller 333 to define a niptherebetween. The second sensor 42 is disposed in proximity to thefourth path 304 and the second reversing roller unit 332 so as to outputa signal for activation of the second reversing roller unit 332 torotate just before the document is moved into the second reversingroller unit 332.

The first guide unit 34 includes a switching gate 341 disposed pivotallyat a junction between the second and fourth paths 302, 304. Theswitching gate 341 is pivotable between a first position shown in FIG. 4b and a second position shown in FIG. 4 c. In the first position, theswitching gate 341 closes the front end of the fourth path 304 to limitmovement of the document into the first reversing roller unit 331 viathe second path 302. In the second position, the switching gate 341closes the second path 302 to limit movement of the document into thefirst reversing roller unit 331 via the fourth path 304.

The ejecting path 305 is provided at a mounting member 401 disposedfixedly in the housing unit, and has upper and lower ends in spatialcommunication with the second and fourth paths 302, 304, respectively.

The third guide unit 46 is disposed at a junction between the ejectingpath 305 and the second path 302 so as to prevent movement of thedocument from the second path 302 onto the ejecting path 305 whileallowing for movement of the document from the ejecting path 305 ontothe second path 302. The fourth guide unit 47 is disposed at a junctionbetween the ejecting path 305 and the fourth path 304 so as to preventmovement of the document from the ejecting path 305 onto the fourth path304 while allowing for movement of the document from the fourth path 304onto the ejecting path 305.

With particular reference to FIG. 4 b, a first-side scanning path isdefined by the feeding tray 311, the first path 301, the second path302, and the first reversing roller unit 331.

With particular reference to FIG. 4 c, a second-side scanning path isdefined by the first reversing roller unit 331, the rear path portion302″ of the second path 302, the third path 303, the lower path portion301″ of the first path 301, the fourth path 304, and the secondreversing roller unit 332.

With particular reference to FIG. 4 d, a document-reversing path isdefined by the second reversing roller unit 332, a rear end portion ofthe fourth path 304, the ejecting path 305, the rear end 3022 of thesecond path 302, the first reversing roller unit 331, and the ejectingtray 30.

A duplex document scanning method using the duplex automatic documentfeeder 3 includes the following steps:

-   (a) moving the document from the feeding tray 311 onto the first    path 301;-   (b) moving the document along the first path 301 to invert the    document to thereby pass through the scanning module 32 so as to    perform a scanning operation on a first side surface of the    document;-   (c) pivoting the switching gate 341 to the first position as a    result of passage of the document through the scanning module 32 as    detected by the first sensor 41;-   (d) moving the document along the second path 302 into the first    reversing roller unit 331 with the second guide unit 45 being pushed    to open the rear path portion 302″ of the second path 302;-   (e) driving the middle roller 333 to rotate counterclockwise as a    result of movement of the document into the first reversing roller    unit 331 as detected by the third sensor 43 to reverse and move the    document from the first reversing roller unit 331 onto the first    path 301 via the third path 303 to invert again the document to    thereby pass again through the scanning module 32 so as to perform a    scanning operation on a second side surface of the document opposite    to the first side surface;-   (f) pivoting the switching gate 341 to the second position as a    result of second passage of the document through the scanning module    32 as detected by the first sensor 41;-   (g) moving the document along the fourth path 304 into the second    reversing roller unit 332 with the fourth guide unit 47 being pushed    to open the fourth path 304 at the junction between the fourth path    304 and the ejecting path 305; and-   (h) driving the middle roller 333 to rotate clockwise as a result of    movement of the document into the first reversing roller unit 331 as    detected by the second sensor 42 to reverse and move the document    from the second reversing roller unit 332 into the first reversing    roller unit 331 via the rear end portion of the fourth path 304, the    ejecting path 305, and the rear end 3022 of the second path 302, to    thereby invert and drop the document onto the ejecting tray 30.

Due to the presence of the fourth path 304, occurrence of paper jams canbe prevented, and the distance traveled by the document is reducedsignificantly, thereby improving the operating efficiency of the feeder3 as compared to that of U.S. Pat. No. 5,784,680. Furthermore, thestructure for changing the position of the switching gate 341 is simplerthan that for moving the lower roller 25 (see FIG. 3) disclosed in USPatent Application Publication No. 2002/0054382.

Alternatively, with reference to FIG. 4 e, the ejecting path 305 isdefined directly by an outer peripheral surface of the middle roller333, and the mounting member 401 is omitted from the housing unit.

FIGS. 5 a, 5 b, 5 c, 5 d, and 5 e show the second preferred embodimentof a duplex automatic document feeder 3′ according to this invention,which is similar in construction to the first preferred embodiment. Inthis embodiment, the second path 302′ is in spatial communication withthe ejecting path 305 at a position under that whereat the second path302′ is in spatial communication with the third path 303. In addition,the third guide unit 46 (see FIG. 4 b) is omitted from the junctionbetween the second and third paths 302′, 303.

A duplex document scanning method using this embodiment is the same asthat using the first preferred embodiment.

FIG. 5 d shows modified first and second reversing roller units 331′,332′. The modified first reversing unit 331′ includes two rollers 3310flanking the rear end 3022′ of the second path 302′ and defining a niptherebetween so as to allow for movement of the document therebetween.The modified second reversing roller unit 332′ includes two rollers 3320flanking the rear end of the fourth path 304 and defining a niptherebetween so as to allow for movement of the document therebetween.

With particular reference to FIG. 5 e, an intermediate path segment 3023of the second path 302′ located between the ejecting path 305 and thethird path 303 is defined by a front wall surface 3024 and a rear wallsurface 3025. To compensate for omission of the third guide unit 46 (seeFIG. 4 b), a top end of the rear wall surface 3025 is higher than thatof the front wall surface 3024 by a distance 71 so as to preventmovement of the document from the rear end 3022′ of the second path 302′to the intermediate path segment 3023 while allowing for movement of thedocument from the intermediate path segment 3023 to the rear end 3022′of the second path 302′.

FIGS. 6 a, 6 b, and 6 c, show the third preferred embodiment of a duplexautomatic document feeder 3′″ according to this invention, which issimilar in construction to the previous preferred embodiments. The maindifference between this embodiment and the previous preferredembodiments resides in that the ejecting path 305 is in spatialcommunication with the junction between the second path 302′″ and thefourth path 304′″.

With particular reference to FIG. 6 d, the first guide unit 34′″ in thisembodiment includes a switching gate 341′″ configured as a curved rod, arotary disk 37, and a solenoid valve 39. The rotary disk 37 is formedwith an integral rotating shaft 38 that is connected fixedly to a middleportion of the switching gate 341′″ and that is disposed rotatably atthe junction between the second and fourth paths 302′″, 304′″. Therotary disk 37 has an outer periphery 371 formed with first, second, andthird positioning notches 372, 372′, 372″. The solenoid valve 39includes an engaging member 391 in the form of a ball. The engagingmember 391 is biased to move into the first positioning notch 372 whenthe switching gate 341′″ is disposed in a first position shown in FIG. 6a, into the second positioning notch 372′ when the switching gate 341′″is disposed in a second position shown in FIG. 6 b, and into the thirdpositioning notch 372″ when the switching gate 341′″ is disposed in athird position shown in FIG. 6 c. The solenoid valve 39 is activated soas to remove the engaging member 391 from the rotary disk 37 just beforethe switching gate 341′″ is pivoted in the housing unit. With particularreference to FIG. 6 a, in the first position, a curved guiding surface3410 of the switching gate 341′″ extends along an intermediate portionof the second path 302′″. When the first scanning operation iscompleted, the switching gate 341′″ is disposed in this position so asto limit movement of the document from the front end 3021′″ of thesecond path 302′″ to the rear end 3022′″ of the second path 302′″.Hence, the document is moved into the first reversing roller unit 331.With particular reference to FIG. 6 b, in the second position, thecurved guiding surface 3410 of the switching gate 341′″ extends betweenthe second and fourth paths 302′″, 304′″. When the second scanningoperation is completed, the switching gate 341′″ is pivoted to thisposition so as to limit movement of the document from the second path302′″ into the second reversing roller unit 332 via the fourth path304′″. With particular reference to FIG. 6 c, in the third position, thecurved guiding surface 3410 of the switching gate 341′″ extends betweenthe ejecting path 305′″ and the fourth path 304′″. When the document ismoved from the second reversing roller unit 332, the switching gate341′″ is pivoted to this position so as to limit movement of thedocument from the fourth path 304′″ into the first reversing roller unit331 via the ejecting path 305′″. Hence, the document is inverted andmoved onto the ejecting tray 30.

With reference to FIG. 6 e, similar to the second preferred embodiment,the first reversing unit 331 may be modified to include two rollers 3310flanking the rear end 3022′″ of the second path 302′″ and defining a niptherebetween so as to allow for movement of the document therebetween,and the second reversing roller unit 332 may be modified to include tworollers 3320 flanking the rear end of the fourth path 304′″ and defininga nip therebetween so as to allow for movement of the documenttherebetween.

In view of the above, in the duplex automatic document feeder, since theupper and lower rollers 334, 335 are not movable relative to the middleroller 333, it is not necessary to provide an elevating mechanism formoving the upper and lower rollers 334, 335, thereby resulting in asimple structure. Furthermore, leading and trailing ends of the documentcannot be clamped between the first or second reversing roller unit 331,332. Thus, occurrence of a paper jam can be avoided.

With this invention thus explained, it is apparent that numerousmodifications and variations can be made without departing from thescope and spirit of this invention. It is therefore intended that thisinvention be limited only as indicated by the appended claims.

1. A duplex automatic document feeder adapted for use in animage-formation device, the image-formation device having a scanningmodule for scanning a document, said duplex automatic document feedercomprising: a feeding tray adapted for feeding the document; a firstpath adapted to extend from said feeding tray to the scanning module soas to invert the document when the document is moved along said firstpath; a feeding/separating roller adapted for moving the document fromsaid feeding tray onto said first path; an ejecting tray; a firstreversing roller unit disposed in proximity to said ejecting tray andadapted for allowing the document to move in a direction onto saidejecting tray and in an opposite direction to reverse the document; asecond path adapted to extend from the scanning module to said firstreversing roller unit and adapted for guiding movement of the documentjust after a first scanning operation is performed on the document; athird path in spatial communication with said first and second paths andadapted for guiding movement of the reversed document from said firstreversing roller unit onto an intermediate portion of said first path; asecond reversing roller unit disposed in proximity to said firstreversing roller unit and adapted for reversing the document; a fourthpath in spatial communication with said second path and said secondreversing roller unit and adapted for guiding movement of the documentjust after a second scanning operation is performed on the document; anejecting path adapted for guiding movement of the document from saidsecond reversing roller unit onto an intermediate portion of said secondpath; and a first guide unit adapted to be controlled to guide movementof the document along a selected one of said second and fourth paths. 2.The duplex automatic document feeder as claimed in claim 1, wherein saidthird path has one end in spatial communication with an end of saidsecond path proximate to said first reversing roller unit, and the otherend in spatial communication with said intermediate portion of saidfirst path.
 3. The duplex automatic document feeder as claimed in claim1, wherein said ejecting path has one end in spatial communication withsaid second path, and the other end in spatial communication with saidfourth path.
 4. The duplex automatic document feeder as claimed in claim1, wherein: said second path has a front end in spatial communicationwith said first path, and a rear end in spatial communication with saidfirst reversing roller unit; and said first reversing roller unitincludes a middle roller and an upper roller flanking said rear end ofsaid second path, said upper roller being disposed above said middleroller to define a nip therebetween so as to allow for movement of thedocument therebetween, a rotational direction of said middle rollerbeing changeable.
 5. The duplex automatic document feeder as claimed inclaim 4, wherein said second reversing roller unit includes said middleroller and a lower roller flanking an end of said fourth path, saidlower roller being disposed under said middle roller to define a niptherebetween so as to allow for movement of the document therebetween.6. The duplex automatic document feeder as claimed in claim 4, whereinsaid ejecting path is defined by an outer peripheral surface of saidmiddle roller.
 7. The duplex automatic document feeder as claimed inclaim 1, wherein: said second path has a front end in spatialcommunication with said first path, and a rear end in spatialcommunication with said first reversing roller unit; said firstreversing roller unit includes two rollers flanking said rear end ofsaid second path and defining a nip therebetween so as to allow formovement of the document therebetween; and said second reversing rollerunit includes two rollers flanking an end of said fourth path anddefining a nip therebetween so as to allow for movement of the documenttherebetween.
 8. The duplex automatic document feeder as claimed inclaim 1, wherein said first guide unit includes a switching gatedisposed pivotally at a junction between said second and fourth paths,said switching gate being pivotable between a first position whereatsaid switching gate closes said fourth path to limit movement of thedocument into said first reversing roller unit via said second path, anda second position whereat said switching gate closes said second path tolimit movement of the document into said first reversing roller unit viasaid fourth path.
 9. The duplex automatic document feeder as claimed inclaim 8, wherein said ejecting path is in spatial communication withsaid junction between said second and fourth paths, said switching gatebeing configured as a curved rod having a curved guiding surface, saidswitching gate being further pivotable to a third position, wherein,when said switching gate is disposed in said first position, said curvedguiding surface guides the document to move from said second path intosaid first reversing roller unit; when said switching gate is disposedin said second position, said curved guiding surface guides the documentto move from said second path into said second reversing roller unit viasaid fourth path; and when said switching gate is disposed in said thirdposition, said curved guiding surface guides the document to move fromsaid ejecting path into said first reversing roller unit.
 10. The duplexautomatic document feeder as claimed in claim 9, further comprising ahousing unit, said switching gate being disposed in said housing unit,said first guide unit further including: a rotary disk disposedpivotally in said housing unit and having an outer periphery formed withfirst, second, and third positioning notches, said switching gate beingconnected fixedly to said rotary disk; and a solenoid valve including anengaging member biased to move into said first positioning notch whensaid switching gate is disposed in said first position, into said secondpositioning notch when said switching gate is disposed in said secondposition, and into said third positioning notch when said switching gateis disposed in said third position, said solenoid valve being activatedso as to remove said engaging member from said switching gate justbefore said switching gate is rotated in said housing unit.
 11. Theduplex automatic document feeder as claimed in claim 1, furthercomprising a sensor disposed in proximity to said fourth path and saidsecond reversing roller unit so as to output a signal for activation ofsaid second reversing roller unit to rotate just before the document ismoved into said second reversing roller unit.
 12. The duplex automaticdocument feeder as claimed in claim 1, further comprising a sensordisposed in proximity to said second path and said first reversingroller unit so as to output a signal for activation of said firstreversing roller unit to rotate just before the document is moved intosaid first reversing roller unit.
 13. The duplex automatic documentfeeder as claimed in claim 9, further comprising a solenoid valve forpivoting said switching gate between said first and second positions.14. The duplex automatic document feeder as claimed in claim 1, whereinsaid second path is in spatial communication with said ejecting path ata position under that whereat said second path is in spatialcommunication with said third path.
 15. The duplex automatic documentfeeder as claimed in claim 2, further comprising a second guide unitadapted to limit movement of the document from said first reversingroller unit onto said first path via said third path.
 16. The duplexautomatic document feeder as claimed in claim 15, wherein said secondpath has a front end in spatial communication with said first path, anda rear end in spatial communication with said first reversing rollerunit, said second guide unit being disposed at a junction between saidsecond and third paths so as to prevent movement of the document fromsaid rear end of said second path to said front end of said second pathalong said second path while allowing for movement of the document fromsaid front end of said second path to said rear end of said second pathalong said second path.
 17. The duplex automatic document feeder asclaimed in claim 3, further comprising a third guide unit disposed at ajunction between said ejecting path and said second path so as toprevent movement of the document from said second path onto saidejecting path while allowing for movement of the document from saidejecting path onto said second path.
 18. The duplex automatic documentfeeder as claimed in claim 17, further comprising a fourth guide unitdisposed at a junction between said ejecting path and said fourth pathso as to prevent movement of the document from said ejecting path ontosaid fourth path while allowing for movement of the document from saidfourth path onto said ejecting path.
 19. A duplex document scanningmethod comprising the steps of: (a) moving a document along a first pathto pass through a scanning module so as to perform a scanning operationon a first side surface of the document; (b) moving the document along asecond path into a first reversing roller unit; (c) driving the firstreversing roller unit to reverse and move the document from the firstreversing roller unit onto the first path via a third path to therebypass again through the scanning module so as to perform a scanningoperation on a second side surface of the document opposite to the firstside surface; (d) moving the document along a fourth path into a secondreversing roller unit; and (e) driving the second reversing roller unitto reverse and move the document from the second reversing roller unitinto the first reversing roller unit via a portion of the fourth path,an ejecting path, and a portion of the second path, to thereby drop ontoan ejecting tray.
 20. The duplex document scanning method as claimed inclaim 19, between the steps (c) and (d), further comprising a step ofmoving a switching gate to a position to prevent movement of thedocument onto the second path while allowing for movement of thedocument onto the fourth path.
 21. The duplex document scanning methodas claimed in claim 19, before said step (a), further comprising a stepof moving the document from a feeding tray onto the first path.
 22. Theduplex document scanning method as claimed in claim 21, where: in saidstep (a), the document is inverted during movement thereof along thefirst path; in said step (c), the document is inverted during movementthereof along the first path; and in said step (e), the document isinverted during movement thereof from the second reversing roller unitinto the first reversing roller unit.